The present invention relates to a semiconductor light emitting element, and more particularly, it relates to a semiconductor light emitting element, such as a light emitting diode (LED), from which the light emitted by supplying a current can be extracted.
The semiconductor light emitting elements have many advantages of such as being compact in size, having low power consumption and being excellent in reliability. Therefore, the semiconductor light emitting elements have been widely used for various uses which demand high emission luminosity such as a display board intended for use indoors or outdoors, a railroad/traffic signal light and various equipment mounted in a vehicle.
For example, the light can be emitted within the broad wavelength range from red to green, by adjusting the composition of the light emitting layer made of InGaAlP quaternary compound semiconductors in the semiconductor light emitting element.
In this specification, the “InGaAlP compound semiconductor” may include all semiconductors having the composition ratios x and y in composition formula InxGayAl1-x-yP in a range of 0≦x≦1, 0≦y≦1, where (x+y)≦1.
Currently, the semiconductor light emitting element using one of gallium nitride (GaN) compound alloy system as a material of the light emitting layer which has the wavelength range from blue to ultraviolet has been developed. Since this material has a band structure of a direct transition, high efficiency of the light extraction can be acquired in the semiconductor light emitting element made of the material. In this specification, the “gallium nitride (GaN) compound semiconductor” may include all semiconductors with the composition ratios x, y and z in a composition formula B1-x-y-zInxAlyGazN in a range of x≦1, y≦1, z≦1, x+y+z≦1. For example, InGaN (x=0.4, y=0, z=0.6) is included in “gallium nitride compound semiconductor”.
FIG. 15 is a schematic diagram showing a cross-sectional structure of a conventional light emitting diode. That is, this light emitting diode has the “double-hetero type” structure where a lower cladding layer 102, an active layer 103, an upper cladding layer 104, and a window layer 105 are laminated in this order on a substrate 101. An upper electrode 106 is provided on the window layer 105, and a lower electrode 107 is provided on the backside of the substrate 101. The current I can be injected into the active layer 103 through these electrodes 106 and 107 as shown by the arrows or in a direction opposite to the arrows.
In the case of “double-hetero type” structure, the upper cladding layer 102 and the lower cladding layer 104 consist of semiconductors which have wider bandgap than the active layer 103. Thus, it is possible to acquire high light emitting efficiency by confining the carriers injected through the electrodes 106 and 107 in the active layer 103.
However, the Inventor has found that in the case of the light emitting diode having such a structure, it might be a problem that a part of emission produced in the active layer 103 could not be extracted outside the element.
FIG. 16 is a schematic diagram for explaining the extraction paths of the emission produced in the active layer 103.
As illustrated in this figure, the emission L11 emitted upward from the active layer 103 can be extracted outside through the cladding layer 104 and the window layer 105. The emission L12 emitted downward in a slanting direction from the active layer 103 can also be extracted outside through the cladding layer 102. The emissions extracted outside the element can be gathered in a predetermined direction by a light condensing means, such as a reflector which is not illustrated.
In contrast, the emission L13 emitted downward from the active layer 103 passes through the cladding layer 102 and the substrate 101, and is reflected by the lower electrode 107. However, there is a problem that the reflectance of the emission L13 is not necessarily high, since an alloyed region is formed near the boundary between the electrode and the semiconductor layer in many semiconductor light emitting elements in order to obtain an ohmic contact. That is, it is not easy to extract the emission L13 emitted towards the lower electrode 107 from the active layer 103 outside with a sufficiently high efficiency since the emission is absorbed in the alloyed region.
The emission L14 which is emitted from the active layer 103 and reaches the side surface of the element with a shallow incident angle may be reflected inside the element. This is because a total reflection of light arises in the case where the refractive index of the material which constitutes the element, such as the substrate 101 is higher than that of the outside media (for example, the atmosphere, nitrogen gas, etc.). If such a total reflection occurs, the problem that the emission L14 is confined inside the element and is absorbed in the alloyed region with the electrode and the active layer 103 and is attenuated arises.
Moreover, the Inventor has found there was a problem that a part of emission produced inside the active layer 103 could not be extracted outside the active layer 103.
FIG. 17 is a schematic diagram for explaining the extraction paths of the emission produced inside the active layer. For example, when the emission is produced in the light emitting point EP inside the active layer 103, the part of the emission L15 emitted upward can be extracted from the active layer 103 through the cladding layer 104. In contrast to this, the part of the emission L16 emitted in a slanting direction inside the active layer 103 from the light emitting point EP may be total-reflected on the interface with the cladding layer 104. This is because the active layer 103 has a higher refractive index than the cladding layer 104.
If such total reflection arises, the emission L16 may be confined and guided inside the active layer 103, and may be lost by an absorption.
The critical angle θc at which such total reflection arises may be about 60 degrees, when the difference between the refractive index of the active layer 103 and that of the cladding layer 104 is 0.5. In such a case, about ⅓ of the light emitted from the light emitting point EP may be total-reflected on the interface with the cladding layer 104, and may be confined and guided inside the active layer, and may be lost by an absorption.
As explained above, in the conventional semiconductor light emitting element, the efficiency with which the light emitted from the active layer is extracted outside is not necessarily high. Furthermore, there has also been a problem that a part of emission produced in the active layer may be confined inside the active layer and be lost.